Traditionally, Flash memories have used a high voltage power supply of 12V (VPP) to produce large electric fields to move charge on and off the floating gate of the Flash memory transistors for program and erase operations. A high voltage, for example, 12V, provides a fast program and/or erase time. The speed of program and erase is one component of the success of flash memory devices. Many memory devices now exist in which 12V is used, and many circuits designed for such memory devices are hardwired with a fixed 12V supply to the memory devices.
However, metal-oxide-semiconductor (MOS) transistors have voltage limitations that depend on gate length, oxide thickness, and doping implants. The presence of high voltages on the gate of a MOS transistor can cause the depletion region between the drain and substrate to pinch along the inner drain-substrate junction, in the channel underneath the gate. For large values of |VDG|, the depletion region is pinched sufficiently to lower the breakdown voltage between the inner drain-substrate pn-junction. This type of breakdown is commonly referred to as gate-aided breakdown of the drain-substrate pn-junction (BVD), and may result in a reverse-bias current flow that could damage the transistor.
In order to improve speed and reduce cost and power, one or more factors (e.g., gate length, oxide thickness, and/or doping implants) are altered. These same factors that affect the speed, cost, and power may also determine the voltage limitations of MOS transistors, and specifically, the BVD limit, or the threshold voltage at which reverse breakdown may occur in a transistor device. The continued drive to improve speed, and reduce cost and power has resulted in MOS transistor devices, such as Flash memories, whose BVD thresholds are lower than the voltage potential of the 12V power supplies traditionally used to power these devices. Thus, circuits that have the entire power supply potential, such as voltage switches, across the circuit face the risk of breakdown because the potential across the circuit exceeds the BVD.